1. Field of the Invention
The present invention relates to a light source assembly using light emitting diodes and a reflective sheet set, and relates more particularly to a light source assembly and the reflective sheet set thereof for the backlight module of a liquid crystal display.
2. Description of the Related Art
Light emitting diodes (LED) have advantages of lower power consumption, high conversion efficiency and extended lifetime, and will not contribute to the environmental problem of mercury pollution. Further, they exhibit excellent color performance when used in a backlight module, and the application of light emitting diodes in display apparatuses has recently become a focus of much research and development.
Based on configuration and position of light source, backlight modules can be divided into two types: a direct-lit backlight configuration and an edge-lit backlight configuration. In the past, the main applications of liquid crystal display have been in notebook computers and liquid crystal display screens. The requirements of such applications for the backlight modules are light weight, low volume, and narrow profile, and the configuration of edge-lit backlight modules can satisfy these requirements. However, when a backlight module for a super large liquid crystal screen is developed, the main considerations are sufficient illumination for the screen, wide viewing angles, high-contrast display, and extended lifetime. In view of the above considerations, development of the direct-lit backlight configuration is of a highest priority.
Referring to FIG. 1, a present liquid crystal display using a direct-lit backlight module as its light source assembly 10 comprises a reflective sheet 11, a plurality of light emitting diodes 12 and a printed circuit board 13. The plurality of light emitting diodes 12 are mounted on the printed circuit board 13. The reflective sheet 11 is also mounted on the printed circuit board 13. The reflective sheet 11 comprises a plate 111 and a plurality of holes 112 formed on the plate 111. The plurality of light emitting diodes 12 are separately situated in the plurality of holes 112, protruding above the holes 112. Each light emitting diode 12 initially emits light into an above-disposed diffusing plate (not shown), producing reflected light. The plate 111 recovers and reuses the reflected light, or reflects the reflected light back to the diffusing plate.
As liquid crystal displays become larger, the dimension of the light source assembly 10 using light emitting diodes and a reflective sheet 11 needs to increase. Regarding the manufacture of the reflective sheet 11, the reflective sheet 11 is made of a metal plate 111, through which a plurality of holes 112 are punched using a punch. However, to meet the precision requirement of the hole positions, the manufacturing cost of the punching mold is high. In addition, reflective sheets 11 having hundreds of holes 112 have higher per-unit costs of manufacture and are difficult to position when they are assembled. For example, if a reflective sheet 11 has some of its holes 112 misaligned or defective, the entire reflective sheet 11 needs to be scraped. In addition, hundred of holes 112 all have to be respectively aligned with the light emitting diodes 12 to accomplish the assembly of the light source assembly 10.
When the light source assembly 10 adopts high power light emitting diodes 12, the high intensity light emitting diodes 12 generate more heat. Heat accumulation may increase the temperature of a printed circuit board, resulting in decreasing intensity of the light emitting diodes 12. Consequently, high intensity light emitting diodes 12 need to be spaced farther apart, causing non-uniform light emission issue of the light source assembly 10. To avoid non-uniformity issue, low power light emitting diodes 12 are used to replace the high power light emitting diodes 12 and are arranged more densely. As a result, the number of holes 112 increases such that the issues of high manufacturing cost of the reflective sheets 11 and the difficulty in assembling the reflective sheets 11 become significant.